Most electronic components, such as integrated circuits (ICs) for example, are sealed within plastic packages. The plastic material is simply molded directly over the IC and a metal lead frame to which it is attached. However, this type of packaging is not particularly well suited for use with MEMS devices, where there is generally a need for an open space within the package to accommodate motion of the mechanical device within. In addition, effective packaging for MEMS and other electro-optical devices often needs to be comprised of hermetically sealed housings to prevent the ingress of corrosive elements such as water vapor and oxygen, isolate internal components from shock and vibration, shield the component from potentially harmful radiation, and provide a means of conducting heat away from power dissipating components. In the case of electro-optic devices, the packaging must also provide a stable platform for the positioning and interconnection of optical components, such as laser diodes, modulators, input and output fibers, and the like.
One of the most important features of a hermetic package is its ability to withstand extended periods of “damp heat” and remain “dry” inside. A typical hermetic test for telecom packages measures the package's ability to withstand 2000 hours in an environment of 85° C. at 85% relative humidity and remain “dry” inside; dry being defined as less than 5000 ppm internal moisture at the end of the test. Materials conventionally used to achieve a hermetic seal are few: metal, glass, and ceramics. Packages sealed properly with these materials are considered truly hermetic. Common hermetic seal interfaces are metal-to-metal seals, made via welding, brazing, or soldering; glass-to-metal seals; ceramic-to-metal seals; and glass-to-glass seals.
An example of a typical hermetic fiberoptic component package is a Kovar box with a Kovar lid that is resistance welded in place via a seam sealer. Light passes in and out of the package via hermetic optical paths. Current methods of passing light through hermetic photonic packages can be categorized as freespace or fiber feedthroughs. Freespace employs hermetic windows having metallized edges that are soldered or brazed into the package wall, sometimes via an intermediate metal ferrule or subcell. A hermetic collimator lens assembly is soldered to a metal package. Telecommunication grade optical fiber typically has a polymer cladding made of UV curable acrylate or Teflon. Hermetic seals cannot be made to these claddings since their moisture barrier properties are inherently low. Hence, wherever optical fiber exits a hermetic package, the cladding layer must be stripped, and the bare silica fiber metallized. Afterwards, a hermetic seal is made to the metallization. Because bare silica fiber is fragile and often breaks, this process is inherently expensive. Hermetic fiber feedthroughs are made using metallized glass fibers that are soldered to the package, typically via a cylindrical sleeve or support that protrudes from the package wall. The ferrule can then be hermetically attached to the package wall, typically soldered.
Fiber feedthrough ferrules with glass frits feature fibers sealed into a metal sleeve via a glass-to-metal seal using a glass frit between the glass fiber and metal sleeve (in this case the fiber is not metallized but its polymer cladding must be stripped). The ferrule can then be soldered to the package wall, or it may be part of the package wall.
Thus, there is a continuing requirement in the industry for low-cost hermetic packages. Polymer packaging would be inherently low-cost; however, adhesives, epoxies, and polymers have not been shown to keep moisture out of packages in extended damp heat tests. Some polymer-based sealing methods and packages may satisfy limited test requirements, but moisture diffuses through these materials over time. Nonhermetic and quasi-hermetic packages are suitable for certain applications. The component end-customer usually determines test requirements.
Recently, a new class of polymers, Liquid Crystal Polymers (LCP), has been shown to have excellent moisture and oxygen barrier properties. Silicon Bandwidth, Inc., (Fremont, Calif.) and Foster Miller, Inc. (U.S. Pat. No. 6,320,257), have both proposed a liquid-crystal polymer package that can be metallized and soldered or welded to suitable lids to produce packages that may not be strictly hermetic but may pass the Telecordia “damp heat” qualification test. Even with an LCP package, the problems of producing an optical port remain; namely, stripping and metallizing the fiber and soldering to the metallization. A need, therefore, exists for an improved technique to implement an optical port to be incorporated into metal, ceramic or LCP packages, and it is a primary object of this invention to satisfy this need.
Additionally, it is known (See U.S. Pat. No. 4,778,244 to Ryan) to overcoat optical fiber with LCPs to provide an optical cable with enhanced strength which, in turn, may be over coated with scuff resistant coatings. However, the scuff resistant coatings described are difficult to remove to allow ready access to the LCP coating for other purposes.
It is another object of this invention to provide optical fiber cable with enhanced strength and easily removable encasing coatings to protect against environmental effects such as moisture and chafing forces.
It is yet another object of this invention to provide optical fiber structures having properties for promoting the formation of hermetic seals when combined with other structures.
It is still another object of the present invention to provide hermetically sealed packaging for optical and electro-optical components.
Yet another object of the present invention is to provide manufacturing processes for fabricating optical fibers coated with LCP and chemically removable encasing layers.
Still another object of the present invention is to provide manufacturing processes by which hermetically sealed devices can be fabricated with LCP materials and optical fibers having supplemental LCP and encasing layers to protect against environmental effects such as moisture and mechanical forces.
Other objects of the invention will, in part, appear hereinafter and will, in part, be obvious when the following detailed description is read in connection with the drawings.